Document all members of bevy_dynamic_plugin (#12029)

# Objective

- Some members of `bevy_dynamic_plugin` are not documented.
- Part of #3492.

## Solution

- Add documentation to members missing it in `bevy_dynamic_plugin`.
- Update existing documentation for clarity and formatting.

---

## Changelog

- Completely document `bevy_dynamic_plugin`.

---------

Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>
Co-authored-by: James Liu <contact@jamessliu.com>

crates/bevy_dynamic_plugin/src/lib.rs

@@ -1,5 +1,23 @@
-// FIXME(3492): remove once docs are ready
-#![allow(missing_docs)]
+//! Bevy's dynamic plugin loading functionality.
+//!
+//! This crate allows loading dynamic libraries (`.dylib`, `.so`) that export a single
+//! [`Plugin`](bevy_app::Plugin). For usage, see [`dynamically_load_plugin`].
+//!
+//! Note that dynamic linking and loading is inherently unsafe because it allows executing foreign
+//! code. Additionally, Rust does not have a stable ABI and may produce
+//! incompatible libraries across Rust versions, or even subsequent compilations. This will not work
+//! well in scenarios such as modding, but can work if the dynamic plugins and the main app are
+//! built at the same time, such as with Downloadable Content (DLC) packs.
+//!
+//! You may be interested in these safer alternatives:
+//!
+//! - [Bevy Assets - Scripting]: Scripting and modding libraries for Bevy
+//! - [Bevy Assets - Development tools]: Hot reloading and other development functionality
+//! - [`stabby`]: Stable Rust ABI
+//!
+//! [Bevy Assets - Scripting]: https://bevyengine.org/assets/#scripting
+//! [Bevy Assets - Development tools]: https://bevyengine.org/assets/#development-tools
+//! [`stabby`]: https://github.com/ZettaScaleLabs/stabby
 
 mod loader;
 

crates/bevy_dynamic_plugin/src/loader.rs

@@ -7,8 +7,10 @@ use bevy_app::{App, CreatePlugin, Plugin};
 /// Errors that can occur when loading a dynamic plugin
 #[derive(Debug, Error)]
 pub enum DynamicPluginLoadError {
+    /// An error occurred when loading a dynamic library.
     #[error("cannot load library for dynamic plugin: {0}")]
     Library(#[source] libloading::Error),
+    /// An error occurred when loading a library without a valid Bevy plugin.
     #[error("dynamic library does not contain a valid Bevy dynamic plugin")]
     Plugin(#[source] libloading::Error),
 }
@@ -18,22 +20,22 @@ pub enum DynamicPluginLoadError {
 ///
 /// # Safety
 ///
-/// The specified plugin must be linked against the exact same libbevy.so as this program.
+/// The specified plugin must be linked against the exact same `libbevy.so` as this program.
 /// In addition the `_bevy_create_plugin` symbol must not be manually created, but instead created
 /// by deriving `DynamicPlugin` on a unit struct implementing [`Plugin`].
 ///
-/// Dynamically loading plugins is orchestrated through dynamic linking. When linking against foreign
-/// code, initialization routines may be run (as well as termination routines when the program exits).
-/// The caller of this function is responsible for ensuring these routines are sound. For more
-/// information, please see the safety section of [`libloading::Library::new`].
+/// Dynamically loading plugins is orchestrated through dynamic linking. When linking against
+/// foreign code, initialization routines may be run (as well as termination routines when the
+/// program exits). The caller of this function is responsible for ensuring these routines are
+/// sound. For more information, please see the safety section of [`libloading::Library::new`].
 pub unsafe fn dynamically_load_plugin<P: AsRef<OsStr>>(
     path: P,
 ) -> Result<(Library, Box<dyn Plugin>), DynamicPluginLoadError> {
     // SAFETY: Caller must follow the safety requirements of Library::new.
     let lib = unsafe { Library::new(path).map_err(DynamicPluginLoadError::Library)? };
 
-    // SAFETY: Loaded plugins are not allowed to specify `_bevy_create_plugin` symbol manually, but must
-    // instead automatically generate it through `DynamicPlugin`.
+    // SAFETY: Loaded plugins are not allowed to specify `_bevy_create_plugin` symbol manually, but
+    // must instead automatically generate it through `DynamicPlugin`.
     let func: Symbol<CreatePlugin> = unsafe {
         lib.get(b"_bevy_create_plugin")
             .map_err(DynamicPluginLoadError::Plugin)?
@@ -46,10 +48,15 @@ pub unsafe fn dynamically_load_plugin<P: AsRef<OsStr>>(
     Ok((lib, plugin))
 }
 
+/// An extension trait for [`App`] that allows loading dynamic plugins.
 pub trait DynamicPluginExt {
+    /// Dynamically links a plugin at the given path, registering the plugin.
+    ///
+    /// For more details, see [`dynamically_load_plugin`].
+    ///
     /// # Safety
     ///
-    /// Same as [`dynamically_load_plugin`].
+    /// See [`dynamically_load_plugin`]'s safety section.
     unsafe fn load_plugin<P: AsRef<OsStr>>(&mut self, path: P) -> &mut Self;
 }